My bag of white cotton insulation on the passenger side floor pan, just under the big rubber foot mat, was in perfect shape, right in front of the huge rust hole in the floor pan. My rust hole looked like it started from the inside and rusted outward, the bag of insulation was perfectly dry and clean.
I just assumed it was sound deadener, probably a later add in, from testing, they found a hot spot where noise was carrying into the compartment and threw down a cheap bag of insulation to kill it.
DYNAMAT would be awesome, but also really really expensive. I'm amazed at some of the resto projects you see them doing on classic cars and they are totally coating the entire floor under the carpet with DYNAMAT, that must have cost $1000. I guess it may be cheaper in bulk packs for that much area, but it still has to be hundreds of bucks.
Lizard Skin is the latest product that is taking off, its like bedliner like Herculiner, but more expensive and better quality. I guess the sound deadening and protection is so good they are applying it to the floor before putting carpet over it.
UnderCoating and some cheap Dynamat substitutes have tar in them, and that gets gooey with heat and flow and mixes with carpet over it, as well as smells. Some of the more expensive, undercoating, 3M Professional is one I used, states it has NO TAR in it. That might work well, I only used that stuff for a small portion of the underbody of my XJ, but it did seem like really good stuff, a medium flat black rubber coat with no smell and felt really dense, tough and adhering to the surface like a well cured paint. If it wasn't $8 a can, I would have done the entire underbody with it.